By widely spreading a mobile device such as a personal computer, a cellular-phone, and a portable video player, the mobile device is used in public space frequently. In this case, anyone can peep the screen of the mobile device. Accordingly, the use of the mobile device in the public place includes problem for security and privacy.
In JP-A H06-118927 (KOKAI), technique for concealment of visual information is proposed. As to this technique, an image to be concealed (Hereafter, it is called “display image”) and an image having inverse color of the display image (it is called “inverse colored image”) are mutually displayed on a screen with time division. By canceling the display image with after-image of the inverse colored image, the display image is concealed for the third party. On the other hand, as for the user side of the mobile device, optical transparence is changed by combining a polarizing direction switching unit with a polarizing filter. The polarizing direction switching unit switches a polarizing direction from the screen in synchronization with time division timing of the display image. Concretely, the user can peruse the display image on the screen on which the image is time-divisionally displayed.
However, when the user peruses a dynamic image on the image display apparatus disclosed in JP-A H06-118927 (KOKAI), a part having brightness largely changed with passage of time (for example, a boundary of a moving object) is viewed as bright (or dark) flicker on the screen. As a result, the third party can estimate the display image from this flicker.
Furthermore, when the user peruses a static image, for example, by switching the present image to another image, or by scrolling the image, the above-mentioned phenomenon often occurs.